Prevention of overvoltages in inverters with controlled semiconductor rectifiers



Jan. 2, 1968 'r. HEHENKAMP 3,36 ,933

PREVENTION OF OVERVOLTAGES IN INVERTERS WITH CONTROLLED SEMICONDUCTOR RECTIFIERS Filed May 12, 1964 1s 21 19 l l F I a i i I I FIG. 2.

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INVENTOR msoooaus HEHENKAMP I nazur United States Patent Ofiice 3,361,933 Patented Jan. 2, 1%68 3,361,933 PREVENTION OF OVERVOLTAGES IN IN- VERTERS WITH CONTROLLED SEMI- CONDUCTOR RECTIMERS Theodor-us Hehenkamp, Emmasingel, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed May 12, 1964, Ser. No. 366,695 Claims priority, application Netherlands, May 14, 1963, 292,750 6 Claims. (Cl. 315-236) ABSTRACT OF THE DISCLQSURE An inverter for powering fluorescent lamps having an inductor with a ferromagnetic core connected in parallel with the lamps. The inductor is designed to have an initial impedance due to the saturation of the core before the lamp ignites which is substantially equal to that of all the lamps being powered, including all the series impedances. When all lamps are operative, the impedance of the inductor is designed to be a multiple of the initial impedance, thereby diverting current through the lamps.

This invention relates generally to inverters for invetting direct current to alternating current and more specifically to such inverters which use controlled semiconductor rectifiers as their switching elements and are provided with a resonant circuit comprising inductance and capacitance.

These inverters are extremely useful for supplying the operating voltage for fluorescent lamps. Examples of such circuits are shown and described in Philips Technical Review, col. 23, 1961/62, pages 272278, and also in the copending application Ser. No. 101,572, filed Apr. 7, 1961, now US. Patent No. 3,229,226, and assigned to the assignee of the instant application.

Fluorescent lamps are gas discharge devices and as such they have the property that when they are switched into circuit they pass substantially no current for at least an initial period of a few milliseconds. The substantially open circuit thus existing across the output of an inverter used for supplying the lamps causes high overvoltages to be produced at said output and across the controlled semiconductor rectifiers. It is evident that these overvoltages can cause serious damage to the circuit components, in particular the rectifiers.

It is a primary object of the invention to reduce by a substantial amount the above-noted overvoltages.

According to one aspect of the invention, an additional element is connected in parallel with the load, this additional element having an initial impedance, i.e. that appearing immediately after the inverter is switched on, which is substantially equal to the impedance of the normal load after the initial period. When the load consisting of the fluorescent lamps has appeared, the additional element has an impedance which is a multiple of the initial impedance. Preferably in accordance with the invention, the additional element comprises an inductor having a ferromagnetic core, and the inductor is connected either directly, or through a transformer, in parallel with the load.

As a result of these measures, the inverter is loaded by the additional element during the initial period, i.e., during the period when it is not normally loaded. This prevents to a considerable extent the undesirable increase in amplitude of the alternating voltages and introduces substantially no losses in the operative condition of the circuit.

The above and further objects of the invention will be better understood from the following description of the invention, when read in conjunction with the accompanying drawing, wherein:

FIG. 1 is a schematic circuit diagram of one embodiment of a circuit arrangement according to the invention;

FIG. 2 is a diagrammatic showing of how the saturation of the ferromagnetic core shown in FIG. 1 may be achieved by direct-curent bias magnetization; and

FIG. 3 is a schematic circuit diagram of an advantageous arrangement for supplying the direct current bias magnetization.

In FIG. 1 a direct-current source 1, 2 is shunted by the series arrangement of an inductor 3 and one of the halves 4 or 5 of the primary winding of a matching transformer 6 through one of the rectifiers 8 or 9, respectively, the latter constituting the controlled semiconductor elements. A capacitor 7 is connected directly in parallel with the primary winding 4, 5. The rectifiers 8 and 9 are rendered alternately conductive by means of suitable control potentials at their control electrodes 10 and 11. In a known manner, the values of the inductance of the inductor 3 and the capacitance of the capacitor 7 are chosen so that the duration of each current pulse through one of the rectifiers is shorter than the duration of a half cycle of the operating frequency of the inverter. The operation is known from the above-noted publications and will therefore not be further described.

Gas discharge or fluorescent lamps 13 are connected as shown to the secondary winding 12 of the transformer 6; a series impedance 14 may be included in series with each lamp 13 for stabilization purposes. As is known, the series impedance 14 may have either an inductive, capacitive or ohmic character.

All gas discharge lamps have the property that when they are switched into circuit they pass substantially no current for an initial period of at least a few milliseconds. This results in a sharp increase in the amplitude of the alternating voltages of the inverter and hence the voltage set up across the rectifiers. This phenomenon is particularly disadvantageous at frequencies exceeding 1000 cycles per second and it must be taken into account when determining the characteristics of the controlled rectifiers to be used; controlled rectifiers designed to withstand the high overvoltages are comparatively expensive.

According to the invention, the load imposed on the inverter by the discharge lamps in normal operation is replaced during the initial starting period of the lamps 13 by an additional inductor 15 having a saturable ferromagnetic core. Inductor 15 and its associated core is arranged to have an initial impedance due to saturation of the core before the lamps ignite which is substantially equal to that of all the normally operating lamps including the series impedances; the inductor 15 is also ar ranged to have an impedance which is a multiple of the said initial impedance at the normal operating voltage when all the lamps are normally operative.

The lamps will not ignite simultaneously. However, each ignited lamp constitutes a part of the total load and hence produces a voltage decrease and consequently an increase in the impedance of the additional inductance 15.

It has been found that with a load consisting of gas and/or vapor discharge tubes, good results are obtained it immediately after the inverter is switched into circuit the impedance of the additional inductor is from to of the impedance of the normal load; after the tubes are ignited, the impedance of the additional inductor should preferably be at least two times the initial value. It has also been found that the core of the additional inductor is saturated to a sufiicient degree to achieve said 3 initial impedance value at a voltage which exceeds the operating voltage by approximately 40%.

In a practical embodiment the direct-current source 1, 2 produced a voltage of about 100 volts; the alternating voltage of the secondary winding 12 had a normal operating value of about 260 volts and was applied to 24 low-pressure mercury vapor discharge lamps 13 each connected in series with an inductive series impedance 14. A voltage of about 108 volts was applied to each lamp and a voltage of about 240 volts to each series impedance 14 with a discharge current of about 0.34 ampere. Thus, each lamp circuit 13, 14 represented an impedance of 260 volts/0.34 ampere or about 770 ohms and all the lamp circuits together represented an impedance of 770 ohms/24 or about 32 ohms. The semiconductor elements 8 and 9 were silicon controlled rectifiers of the pnpn type. The operating frequency was about 7000 cycles per second. The additional inductance 15 had an impedance of about 40 ohms at 300 volts and about 400 ohms at 260 volts.

When the inductance 15 was used,a maximum voltage of about 380 volts was applied to the rectifiers 8, 9 before the ignition of the lamps 13; when the inductance 15 was omitted this voltage increased to about 450 volts. A voltage of the latter value requires the use of considerably more expensive rectifiers or the replacement of each rectifier by two series-connected rectifiers.

It is evident that the inverter is also loaded by the no-load voltage of the matching transformer 6 before the lamps ignite. However, this impedance is generally much too high to achieve by itself an appreciable reduction of the voltages produced before the lamps have become operative. In the example given above, this impedance was about 600 ohms.

The saturation of the ferromagnetic core of the inductor 15 is accomplished in FIG. 1 in response to the A.C. current passing through the coil. It may alternatively be obtained by direct. current bias magnetization of the core.

An example of direct current bias magnetization of the core is shown in FIG. 2. As therein shown, the alternating current winding 15 of FIG. 1 may be divided into two equal windings 18 and 19 which are arranged on legs 16 and 17 respectively of a shell-type core and are connected so that the alternating current flux in a center leg 30 is canceled. A directcurrent Winding 21 is wound on the center leg and is connected to a suitable source of direct voltage to provide the bias magnetization desired. By proper adjustment of the DC bias, the impedance of the inductance 15, when the inverter is switched into circuit, may be made to be substantially equal to the impedance of the load in the normal operating condition of the circuit.

PEG. 3 shows an advantageous circuit arrangement for supplying the direct-current bias for the winding 21. In this figure, circuit elements designated by reference numerals 22 to 29 are coupled to the anode and cathode of controlled rectifier 9 as shown in order to energize direct-current winding 21, the remainder of the inverter circuit remaining the same and therefore not being shown. As shown in FIG. 3, the direct-current winding 21 is connected through a resistor 22 to a capacitor 23. When the inverter is switched into circuit, capacitor 23 is very rapidly charged through resistor 24 and controlled rectifier 25. This may be facilitated by supplying a control current to the control electrode 26 through a rectifier 27; as can be seen, the latter is connected in series with a capacitor 23 and a resistor 29. When the rectifier 9 becomes conductive, the rectifier 25 becomes nonconductive and remains in this condition because the capacitor 28 remains charged. Thus, the charging circuit for capacitor 23 is broken and this capacitor discharges through the direct-current winding 21 and provides the bias for the ferromagnetic core.

Various modifications and changes in the abovedescribed embodiments will be apparent to those skilled in the art without departing from the inventive concept, the scope of which is set forth in the appended claims. It should also be understood that any quantitative values are given for illustrative purposes and only to enable ready practice of the invention.

What is claim is:

1. A circuit arrangementv for feeding an alternating voltage to a load, comprising: an inverter for supplying alternating current including at least one controlled semiconductor rectifier and having a resonant circuit coupled therewith, said inverter further including direct current input terminals adapted to be connected to a direct current supply source and alternating current output terminals,

said rectifier and resonant circuit being connected between said input and output terminals, a load coupled to said output terminals, said load appearing an initial period of time after the inverter begins developing alternating voltage across said load, said load being substantially nonconducting during said period of time, and an inductor with a ferromagnetic core connected in parallel with the load, the impedance of said inductor after the inverter starts operation and prior to the expiration of said period having an initial value causing said alternating current to be diverted through said inductor during the nonconducting period of said load, the impedance of the inductor having a value after said initial period which is a multiple of said initial value, thereby diverting said alternating current to said load.

2. A circuit is claimed in claim 1, wherein said load comprises gas and/or vapor discharge tubes and the impedance of the inductor has an initial value having a range from to of the load impedance after said initial period and a value at least ten times the initial value after said initial period.

3. A circuit arrangement for feeding an alternating voltage to a load, comprising: an inverter for supplying alternating current including at least one controlled semiconductor rectifier and having a resonant circuit coupled therewith, said inverter further including direct current input terminals adapted to be connected to a direct current supply source and alternating current output terminals, said rectifier and resonant circuit being connected between said input and output terminals, a load coupled to said output terminals, said load appearing an initial period of time after the inverter begins developing alternating voltage across said load, said load being substantially nonconducting during said period of time, and an inductor with a saturable ferromagnetic core connected in parallel with the load, said core being saturated at a voltage exceeding the normal operating voltage by a maximum of 40%, the impedance of said inductor after the inverter starts operation and prior to the expiration of said period and upon saturation of the core having an initial value causing said alternating current to be diverted through said inductor during the nonconducting period of said load, the impedance of the inductor having a value after said initial period which is a multiple of said initial value, thereby diverting said alternating current to said load.

4. A circuit arrangement for feeding an alternating voltage to a load, comprising: an inverter for supplying alternating current including at least one controlled semiconductor rectifier and having a resonant circuit coupled therewith, said inverter further including direct current input terminals adapted to be connected to a direct current supply source and alternating current output terminals, said rectifier and resonant circuit being coupled between said input and output terminals, a load coupled to said output terminals, said load appearing an initial period of time after the inverter begins developing alternating voltage across said load, said load being substantially nonconducting during said period of time, and an inductor with a saturable ferromagnetic core connected in parallel with the load, means for producing a direct current bias for biasing said core to saturation during said'initial period, the impedance of said inductor after the inverter starts operation and prior to the expiration of said period and upon saturation of the core having an initial value which is substantially equal to the impedance of the load after the expiration of the time period for causing said alternating current to be diverted through said inductor during the nonconducting period of said load, and means for removing said bias after the initial period for reverting the impedance of the inductor to a value after said initial period which is a multiple of said initial value and thereby diverting said alternating current to said load.

5. A circuit arrangement as claimed in claim 4, wherein said core is saturated at a voltage exceeding the normal operating voltage by a maximum of 40%.

6. A circuit arrangement for feeding an alternating voltage to a load, comprising: a direct current to alternating current inverter including at least one controlled semiconductor rectifier and having a resonant circuit coupled therewith, said inverter having a direct current input and an alternating current output, the controlled semiconductor rectifier together With the resonant circuit being coupled between said input and output, said load being coupled to said output, and an inductor having a ferromagnetic core connected in parallel with said load, wherein initiation of inverter operation develops an alternating voltage across said load and inductor, said load being substantially nonconducting during a short initial period of time and said inductor having a low impedance during said period of time whereby said alternating current is diverted through said inductor, the decrease of conductivity of said load after said initial period of time reverting the impedance of said inductor to a relatively high value and thereby diverting said alternating current to said load.

References Cited UNITED STATES PATENTS 2,949,565 8/1960 Rohloif et al. 3l5200.1 3,222,573 12/1965 Lord 32382 X 3,227,921 1/1966 Sanderson 315-l00 3,263,122 7/1966 Genuit 315- JOHN W. HUCKERT, Primary Examiner. R. F. POLISSACK, Assistant Examiner. 

